Phase-locked loops have been known for many years and can be described broadly for use in most applications as comprising an electronic servo loop consisting of a phase detector, a low pass filter, and a voltage controlled oscillator. By controlling the phase of the oscillator output signal, the loop is capable of locking to, or synchronizing with, the phase of an incoming signal.
In one application, for example, a phase-locked loop can be utilized to provide suitable clock signals in a system for controlling the positioning of a magnetic head with respect to a magnetic media, such as a magnetic disk or drum memory storage apparatus, in a data processing system. In such an application the input signal from the data tracks of the magnetic disk, for example, may assume a plurality of frequencies which are related by a known factor such that they are fixed multiples of each other. It is desirable to generate a clock signal having a frequency which is an integral multiple of the frequencies assumed by the input signal, and such that the input signal is also always maintained in phase with the input signal no matter which one of the plurality of frequencies such input signal assumes. In such application it is desirable that a phase-locked loop responsive to the input signal be utilized to provide such a clock signal, the frequency and phase of which is always locked to that of the input signal.
In conventional phase-locked loops using voltage controlled oscilllators, the frequency range centered about the initial free-running frequency of the voltage controlled oscillator over which the loop can acquire lock with an input signal is normally designated as the capture range. In assuring that the loop can capture the phase of the incoming signal, phase-locked loops of the prior art have used many different techniques to adjust the circuit operation so as to provide an appropriate capture range. For example, in many cases a potentiometer is utilized to adjust the control voltage of the voltage controlled oscillator for operation near its center frequency. In other cases tightly controlled elements, such as inductors and capacitors, and the like, are utilized to assure that the voltage controlled oscillator free-running frequency is appropriately designed to provide a suitable capture range. Other systems utilize specially designed circuitry, such as ramp generators, for sweeping the voltage controlled oscillator through a capture range, while still other circuitry uses injection locking techniques wherein the input data is injected directly into the voltage controlled oscillator circuitry via an inductor/capacitor tank circuit. In the latter case, beating the tank circuit initiates the oscillator circuit to operate at the data frequency so that the phase-locking feedback loop can then be closed to maintain a lock-in condition. A very complex structure for performing the latter operation, for example, is disclosed in U.S. Pat. No. 3,810,234, issued to M. R. Monett on May 7, 1974.
In solving the capture range problem in the prior art, the circuitry involved often necessitates the use of expensive parts and highly stable components, the selection and use of which adds to the overall complexity of design as well as to the cost of manufacture in order to achieve the desired reliability of the system. In contrast, it is desirable to provide for a phase-locked loop which achieves automatic capture but which eliminates the need for additional expensive components and complex circuitry so that the overall design is relatively simple in form and can be relatively easy and less costly to manufacture without sacrificing the reliability of performance that is desired.